The following pertains to the nuclear reactor arts, nuclear power arts, nuclear reactor safety arts, radiological release containment arts, and related arts.
Nuclear power plants incorporate safety systems to continue radioactive reactor core cooling in the event of a safety-related reactor shutdown. These systems are commonly referred to as emergency core cooling (ECC) systems. During a safety-related reactor shutdown, the nuclear chain reaction operating on the fissile isotope (235U in the case of typical light water reactors employing enriched uranium fuel) is terminated almost immediately by release (or “SCRAM”) of neutron-absorbing control rods which are arranged to fall into the nuclear reactor core under gravity. The shutdown provided by the control rods is supplemented in some reactor designs by injection of a soluble neutron poison (typically a soluble boron compound) into the reactor pressure vessel. However, after termination of the nuclear chain reaction by the SCRAM, the radioactive core continues to generate residual decay heat due to continuing decay of unstable isotopes which were formed in the core as intermediate products of the nuclear chain reaction. These unstable isotopes have half-lives for spontaneous decay on the order of minutes, hours, days, or longer, and the residual decay heat from these radioactive isotopes is accommodated by the ECC system. Active ECC system designs employ pumps to inject water to provide the emergency core cooling, with batteries and/or standby diesel generators providing backup power. Passive ECC system designs rely on automatic depressurization to lower reactor coolant system (RCS) pressure to allow passive injection of water. If the ECC system operates as designed, damage to the nuclear reactor core is minimized or prevented completely.
In the United States, the Nuclear Regulatory Commission (NRC) requires that every nuclear power plant incorporate additional safety systems designed to operate in the event of an ECC system failure. Other nuclear regulatory jurisdictions typically have similar regulations. In an ECC system failure, residual decay heat is not removed in an effective manner, and the nuclear fuel assemblies and surrounding steel structures may melt and form a molten mass sometimes referred to as “corium” that relocates to the bottom of the reactor pressure vessel. During nuclear meltdown, the most severe type of nuclear reactor failure, the high temperature of the corium (UO2 melts around 3100K) may be sufficient to cause the corium to further melt through the bottom of the reactor vessel and relocate to the floor of the radiological containment structure. To accommodate such an ex vessel retention scenario, the floor of the radiological containment beneath the reactor pressure vessel is lined with high temperature insulating tiles, for example made of zirconium dioxide or zirconia, to minimize interaction of the corium with the concrete. The lower portion of the reactor pressure vessel is typically located in a cavity in the containment floor, which is filled with water. The molten core is allowed to spread out over the cavity floor to a more readily cooled geometry.